This research is concerned with the principles controlling the evocation of simple responses and is guided by a form of decision theory in which variability of the criterion plays a dynamic role. A major goal is to determine by scaling procedures the functions of time describing the processing of sensory and associative information. The criterion is related to variables determining motivation, attention, adaptation, and set. Response latency and probability are determined by sensory and associative strength in relation to criterion level and variability. Methods include simple, disjunctive, and choice reaction time. Examples of experimental variables include stimulus intensity, stimulus similarity, stimulus probability, and individual differences.